1. Field of the Invention
The present invention relates to a robot manipulated roll hem forming apparatus.
2. Background Art
Roll hem forming is a production process that is used to join a sheet metal outer panel to an inner panel by forming a hem flange. The hem flange is formed by bending a peripheral flange of an outer panel from a generally perpendicular orientation to a reversely turned orientation over an outwardly extending flange of an inner, reinforcement panel. A robot may be used to manipulate an end effector tool having a roller that is rolled against the peripheral flange of the outer panel to cause it to overlie the outwardly extending flange of the inner panel. In a robotic hemming system, the robot manipulates the roller about the edges to be hemmed.
Hard robot systems use precise position control of the location of the roller to press the outer panel peripheral flange against an anvil or solid supporting surface. A principal problem associated with hard robot systems is that they require precise position programming and lack flexibility. The end effector roller must be exactly located within about 0.1 mm or panel quality problems may be encountered due to application of excessive or inadequate force to the hem flange. The range of force applied to properly form an aluminum hem is between 150 and 350 psi. Other materials such as steel or specialized alloys would have different ranges of acceptable force application. A hem may be inadequately formed if insufficient force is applied resulting in a partially formed hem and potential for movement of the inner flange. If excessive force is applied to the hem, the peripheral flange of the outer panel may split or otherwise be distorted. Robots that are capable of meeting these exacting requirements are more expensive than standard robots.
Force control robot systems have been proposed in which a force transducer, or spring element, is incorporated into the end effector to address problems associated with hard robotic hemming systems. The force transducer moderates the force applied by the robot arm to the hem by the roller. Force control systems allow for limited misalignment of the hemming roller relative to the anvil. Examples of force transducers used in force control systems include mechanical springs, hydraulic dampers and air springs. A disadvantage associated with conventional force control systems is that the roller must generally be in line with the force transducer and robot arm or an angular deflection error will arise.
One example of a force control system is disclosed in U.S. Pat. No. 5,228,190 in which a slide block is incorporated in the end effector to connect the roller to a mechanical spring biasing member. Another example of a force control system is disclosed in U.S. Pat. No. 5,038,597 that discloses an end effector incorporating a parallelogram linkage that engages a resilient biasing element to bias the roller to a starting position. A hydraulic cylinder is provided to extend and retract the roller relative to the workpiece. With either of these prior art systems, only a single roller is provided even though in many hemming operations long straight hems are performed more quickly by a large diameter roller. Tight radius hems and hems in confined locations are preferably formed by means of smaller diameter rollers. It is necessary to change out the end effector to provide large and small rollers to hem panels requiring large and small rollers in such prior art force controlled robotic hemming systems. Changing out an end effector is time consuming and increases the cycle time of the hemming operation.
Another disadvantage of conventional systems is that the robot must extensively articulate the end effector to position the roller relative to the anvil surface and flanges to be hemmed. This problem is associated with both hard robotic hemming systems and force controlled hemming systems. Generally, a single roller must be moved by the robot to different angular orientations for a pre-hemming step and a final hemming step. In the pre-hemming step, the peripheral flange of the outer panel is formed to an acute angle of about 45° or may be formed in a two-step process to acute angles of 30° and then 60° to reduce forming stresses applied to the hem. Moving the roller to different angular orientations generally requires complex manipulations of the robot arm that increase cycle time.
There is a need for an improved hem roller end effector that includes a force transducer and a plurality of different rollers to provide an appropriately sized roller for different hem areas and that also may provide rollers oriented at different angular orientations relative to the end of the robot arm. The above problems and needs are addressed by Applicants' invention as summarized below.